For some time now, it has been common practice to produce manufactures with anti-bacterial and anti-pollution properties destined for the building industry with a wide range of applications such as, construction conglomerates, panelling for road networks and self-locking blocks for paving.
These manufactures are constituted, substantially, of a cement-based conglomerate whose mass contains particles of titanium dioxide, TiO2, a property of which is its capacity to reduce the polluting agents present in the surrounding air.
Of these polluting agents, particular attention should be focussed on the polycyclic aromatic hydrocarbons (PAH) derived from the incomplete combustion of organic materials, wood, coal, oil, and its derivatives, and also the nitrogen oxides (NOx) present in the exhaust fumes from heating systems, industrial plants, motor vehicles, industrial wastes and pesticides. The abatement of the level of environmental NOx reduces both the possibility of acid rain and the presence of nitrates which are harmful to humans and vegetation. The bacteria which can be attacked by the presence of TiO2 include, for example, Staphylococci and Escheria Coli. This result is due to the fact that the ultraviolet radiation of solar light, together with the humidity, interacts with the titanium dioxide particles, leading to the production of active oxygen which effectively oxidises the aforesaid polluting and bacterial agents present in the atmosphere. The products of the aforesaid oxidisation are removed by water, either rainwater or washing water, and also by the alkaline nature of the cement conglomerates which, until now, have been realised with photocatalytic properties. Moreover, the aforesaid removal and elimination of the polluting products prevent their stagnation on the surface of the said manufactures, ensuring the original colours and their attractive appearance are maintained over time.
The drawback of the commonly-known manufactures with photocatalytic properties lies in the cement base of the said products which cause the said manufactures to have a rough surface and, consequently, limited possibility of high quality aesthetic variants and, consequently, limited possibility of application in the field of external wall coverings for the building industry.
Much higher aesthetic quality, for external wall coverings, is offered by manufactures of a ceramic nature, in particular porcelain stoneware tiles or those made of other pastes, such as single-fired or monoporous materials; however, until now, as far as the applicant is aware, no such material has been realised with polluting and antibacterial agent abatement properties. The main reason for this lies in the fact that the production of ceramic tiles requires firing treatments involving extremely high temperatures which, in the case of the porcelain stoneware manufactures, reach 1,200° C., unlike the production cycle of cement-based manufactures, which are produced by mixing and subsequent compaction, without requiring any thermal treatments.
It is known that when the TiO2 in the form of Anatase reaches 900° C., it is transformed entirely into Rutile, which, it has been demonstrated through experimentation, to be less effective than Anatase in particularly critical pollution situations such as on roads with a great deal of traffic. Moreover, it has been observed that, in the production phase, the transformation of Anatase into Rutile gives the glaze on ceramic manufactures a yellowish colour.
The applicant's studies have concentrated on the way in which the TiO2 can be made to coexist with the aforesaid extremely high temperatures without the decay of its photocatalytic properties, in addition to ensuring the photocatalytic effect of the ceramic tiles obtained in this way do not cause a decline in the high aesthetic quality of the said tiles.
The applicant's research and experiments have lead to different considerations that are analysed hereunder.
The total presence of TiO2 in a finished tile (as a percentage thereof) which provides the best results varies within a range of 1-25% of the total weight of the applications (glazes, silk-screening, engobe, etc.). It should be highlighted that the photocatalytic reaction of the TiO2 to the polluting and bacterial agents does not involve the consumption of the said TiO2, which means the efficacy of its action remains constant over time. It should also be noted that TiO2 can be mixed with both the glaze and the engobe (a covering formed of a thin layer of atomised clay applied to the partially dried clay) and can also be applied with the silk-screening pastes. The effectiveness of the oxidisation exerted by the TiO2 on the bacterial and polluting agents increases in the event that a photo-reflecting layer is applied, either beneath the layer of glaze covering the tile or with the said covering layer; for example, both white pigments and silica particles mixed with the glaze can be used; in this way, the rate at which the solar light penetrates the tiles is increased and this increases the photocatalytic effect exerted by the TiO2.
It is known that TiO2 converts NOx into nitrate ions which, upon oxidisation, become Sodium and Calcium nitrates, which are not noxious, and which precipitate in the form of salts; the latter are removable by simply washing with water. From this came the idea that this washing action would be increased by an increase in the exposed surface, therefore the creation of micro channels on the surface of the tile would facilitate the action of the water (rain water or washing water) when removing the products of the oxidisation caused by the pollutants.
In parallel, it was observed that the increase in the surface exposed to the light increases the photocatalytic effect of TiO2 and therefore it appeared evident to the applicant that it was also necessary to create, on the surface of the tiles, a plurality of non-uniform, micro uneven areas with the dual aim of permitting the solar light to hit the tiles from any direction and permit the air to better fix the NOx which is decomposed by the ultraviolet radiation of the daylight.
To permit an efficacious retaining action of the gas developed during the night, while awaiting the daylight, the need to provide the tiles with materials able to store the said gas was considered. To this end, experimentation showed that the materials that absorb well are Zeolite and Petalite minxed with the glaze, or Magalite added to the traditional clays of which the tile base is composed.